Many applications such as pulsed-laser diagnostics, laser radar, and optical signal processing utilize information contained in modulations of the intensity envelope of optical pulses. Intensity modulations can be directly measured on time scales longer than approximately 10 ps with a high-speed photodetector and a sampling oscilloscope. However, for optical pulses shorter than a few picoseconds, the electrical signal generated by the high-speed photodetector is dispersed by the transmission line that couples the photodetector to the sampling receiver of the oscilloscope and the information is scrambled or lost completely.
Other techniques for measuring high-speed intensity modulation have been proposed. One technique utilizes two photoconductors separated by a length of transmission line to correlate the optical pulses. The temporal resolution of the correlation, however, is limited to several picoseconds because of linear dispersion of the electrical pulses in the transmission line.
Another technique for measuring high-speed intensity modulation utilizes a photodiode connected to a high-speed microwave detector with a short length of transmission line. The optical pulses are converted to electrical pulses by the photodiode. The electrical pulses then propagate along the transmission line and are cross-correlated by the microwave detector. The temporal resolution of this technique is also limited to several picoseconds because of linear dispersion along the transmission line and because of parasitic capacitance in the photodiode.
Streak cameras have been utilized in laboratory experiments to measure intensity modulations with temporal resolution of approximately one picosecond. Steak cameras, however, not practical for use in most system applications.
Sub-picosecond correlation techniques have been developed that utilize nonlinear crystals to measure intensity modulation. These correlation techniques measure the nth-order intensity autocorrelation function for orders of n=2,3. For example, autocorrelation techniques that utilize nonlinear crystals for second-harmonic generation measure the second-order intensity autocorrelation function: ##EQU1## These autocorrelation techniques, however, are notoriously sensitive to wavelength, polarization, intensity, and orientation of the input beams with respect to the crystal because of strict phase-matching requirements.
There currently exists a need for an all-solid-state ultrafast correlator that can be monolithically integrated and that is relatively insensitive to wavelength, polarization, intensity, and orientation of the input beams.